The present invention relates to a line head using organic EL (electroluminescence) elements and an image forming apparatus incorporating such a line head.
There has been developed image forming apparatus in which a line head having a large number of arrayed light emitting devices is used as an exposure unit. Japanese Patent Publication No. 11-138899A discloses an image forming apparatus in which a light emitting device array made of a plurality of light emitting devices is integrated in a single chip so as to form an exposure unit. In this example, single-chip light emitting device arrays for respective colors are once formed on a single substrate and then separated. The single-chip light emitting device arrays are disposed in corresponding color developing units respectively. Thus, the variation in light emitting characteristic is canceled.
In addition to light emission diodes (LEDs), organic EL elements have been proposed as light emitting devices applicable to such a line head. There is an advantage that the organic EL elements can be controlled statically so that a control system therefor can be simplified. In a line head using a plurality of light emitting devices made of organic EL elements, the light emitting devices may be driven by constant current control or constant voltage control in order to keep the quantities of light emission from the light emitting devices constant.
In a case where the light emitting devices are often driven by constant voltage control with a comparatively simple circuit configuration, the operating time period may exceed a predetermined time period when light emitting devices made of organic EL elements are driven by constant voltage control. In such a case, it has been known that the emitted light amount from each light emitting device is lowered due to a change in resistance value of the light emitting device caused by the intrinsic characteristic proper to the organic EL element.
Assume that a constant voltage Va is applied to each light emitting device as shown in FIG. 17A. In this case, the emitted light amount has a constant value Ia till the operating time period reaches a predetermined time period or 200 hours in this example, as shown in FIG. 17B. When the operating time period exceeds 200 hours, the emitted light amount begins to decrease from the value Ia to a value Ix. When the operating time period exceeds 250 hours, the emitted light amount further decreases to a value Iy.
Japanese Patent Publication No. 11-138899A also teaches the use of organic EL elements as light emitting devices. However, there is no suggestion about a solution to the lowering of the emitted light amount when the operating time period exceeds a predetermined value as shown in FIGS. 17A and 17B in the case where organic EL elements are controlled by constant voltage control. Therefore, when organic EL elements attached to a line head are driven by constant voltage control, there is a problem that the technique cannot deal with deterioration in image quality caused by the lowering of the emitted light amount when the operating time period exceeds the predetermined value.